A question of attribution

On the day the SPM was published, I wrote about the key IPCC claim that it was “extremely likely” (i.e. at least 95% certain) that human influence had caused most of the warming since 1950. This was an increase on the AR4 attribution statement of 90%, which seemed odd in the light of another six years with no warming, contrary to climate science predictions, and increasing number of papers from mainstream climate scientists acknowledging a discrepancy between models and observations. However, a direct comparison is not appropriate since the IPCC moved the goalposts from greenhouse gases to all human influences. At the time of that post the justification for the 95% claim was not available, but the SPM referred to Chapter 10 of the main report, “Detection and Attribution of Climate Change” in particular section 10.3.

A post at Realclimate tries to address the attribution question but does little more than re-state the IPCC claim, based on another IPCC figure and its error bars.

A post at the manicbeancounter blog notes that the uncertainty levels for CO2 forcing has increased greatly between between AR4 and AR5, which seems inconsistent with the claim of increased certainty.

Apart from these two, I can’t find much detailed investigation of the 95% claim and how it arises from the main report. Are there other blogs I missed?

Chapter 10 starts off with the statement that “More than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010 is very likely due to the observed anthropogenic increase in greenhouse gas concentrations.” This is almost exactly the same claim as made in AR4. So it seems that the inflation from 90% to 95% results from the switch from greenhouse gases to total anthropogenic contribution.

The second paragraph states the 95% claim in the SPM, with slightly different wording: “It is extremely likely that human activities caused more than half of the observed increase in global mean surface temperature from 1951–2010.”

Here is figure 10.5, quoted at RC. (My screen-grabber has again garbled the colours, so apologies for any confusion)

It shows the observed warming trend at the top, followed by the trend attributable to greenhouse gases, the trend attributed to total anthropogenic factors, and the (cooling) trend due to man-made aerosols. Each of these has error bars associated with it. There is a very obvious problem with this figure, pointed out by Clive Best in his comment at RC:

“Can you explain why in figure 10.5 the error bar on ANT is so small? Naively I would expect this to be the sum of GHG and OA. This would then work out to be an error on ANT of sqrt(2*0.36) = 0.8C. This is also not explained in chapter 10.”

I had a look through the SOD version of Chapter 10, the version reviewed by scientists, to see if this strange figure was there – it isn’t.

The only way the figure makes any sense is if the IPCC has decided a priori that almost all the warming must be anthropogenic, because they can’t think of anything else, but are uncertain how to divide that warming into greenhouse and aerosol effects.

But even this explanation does not make sense, in view of this figure from Chapter 8, showing the total “effective radiative forcing” of GHGs, aerosols and the total anthropogenic.

In this case the error bars add up in a sensible way, as suggested by Clive – the error bar for total anthropogenic forcing is greater than that either GHGs or aerosols.

So the 95% claim seems to be based on a nonsensical figure that is contradicted by another graph elsewhere in the IPCC report.

[Culture spot: “A question of attribution” is a play by Alan Bennett about Anthony Blunt, custodian of the Queen’s pictures and Russian spy. According to Wikipedia, Bennett described the piece as an “inquiry in which the circumstances are imaginary but the pictures are real.” There is a great TV version with James Fox and Prunella Scales.]

8 thoughts on “A question of attribution”

Matthews is confusing the estimates of radiative forcing since 1750 with a completely separate calculation of the best fits to the response for 1951-2010.

I don’t see any reference in your post to radiative forcing since 1750 – nor does Fig 8.16 for that matter.

Here is a simply example of how it works. Let’s say that models predict that the response to greenhouse gases is A+b and to aerosols is -(A+c). The “A” part is a common response to both, while the ‘b’ and ‘c’ components are smaller in magnitude and reflect the specific details of the physics and response.

Hang on there. A for GHG goes something like 5.3ln(C/C0), whereas A for anthropogenic aerosols is completely different and depends on black carbon, SO2 , NO2 etc. I can’t see why they should be equal and opposite because CO2 emissions have changed dramatically from coal based to oil & gas based since 1950. More important is that the increased aerosols in the last 30 years is mainly due to deforestation and land clearance decoupled from simply burning fossil fuels.

The aerosol pattern is negative (i.e. cooling). The total response is expected to be roughly X*(A+b)-Y*(A+c) (i.e. some factor X for the GHGs and some factor Y for the aerosols). This is equivalent to (X-Y)*A + some smaller terms. Thus if the real world pattern is d*A + e (again with ‘e’ being smaller in magnitude), an attribution study will conclude that (X-Y) ~= d. Now since ‘b’ and ‘c’ and ‘e’ are relatively small, the errors in determining X and Y independently are higher. This is completely different to the situation where you try and determine X and Y from the bottom up without going via the fingerprints (A+b or A+c) or observations (A+d) at all.

Again this depends on A being constant between CO2 and aerosols which is not really apparent and b, c and e being small.

Really Peter Stott from the Hadley centre should explain the difference between Fig. 8.16 and Fig. 10.5 and justify the small error on ANT.

Thanks Clive, his response is complete nonsense, as you say.. Posting a link to here from RC was a curious tactic – Gavin’s comment is #41 on the RC thread. There’s a follow-up comment in #50 that gets more BS from Gavin in reply:

Just for completeness, and to preempt any confusion, …

Nope, you’ve only made things worse. Is A chose arbitrarily, or does it mean something? Can we choose A = 0? Or can we maximise A, so that either b or c is zero? Why are b and c small?

[Response: Sorry! Basically, the A+b/A+c thing and their magnitudes is a rough summary of the results. The commonality of the response is related to the commonality of the response to all near-global, near-homogenous forcings – the feedbacks that get invoked (water vapour, ice-albedo, clouds) are similar in both cases (though with differences in the regions where there is stronger inhomogeneity). If the fingerprints were completely orthogonal, it would be the ‘A’ part that would be small – that isn’t the case though. – gavin]

Comment by Greg Simpson

and there’s further confusion from Patrick027 in #54.

My twitter feed is also full of Schmidt this morning, starting with

@clivehbest @etzpcm You are confusing yourselves. The err. bar in total forcing != uncertainty in pattern of response (and scaling to ons).

So he thinks that it’s perfectly fine to have a wide pdf for forcing and a narrow one for the temperature response! Your final tweet

Figure 8.16: Probability density function (PDF) of ERF due to total GHG, aerosol forcing, and total anthropogenic forcing. The GHG consists of WMGHG, ozone, and stratospheric water vapour. The PDFs are generated based on uncertainties provided in Table 8.6.

and the caption to Table 8.6 says:

Table 8.6: Summary table of RF estimates for AR5 and comparison with the 3 previous IPCC assessment reports. ERF values for AR5 are included. For AR5 the values are given for the period 1750–2011…

Also, at the top of the chapter, the second bullet of the Executive Summary (page 8-3) says:

RF and ERF are estimated over the industrial era from 1750 to 2011 if other periods are not explicitly stated.

It was also conventional to define RF relative to 1750 in AR4, which I know Paul read because we talked about it on Bishop Hill a couple of years ago. So, it should not be a surprise that RF is again defined relative to 1750 in AR5, and it is very clearly stated both at the very start of the chapter and in the information that went into Fig 8.16.

But that aside….

Paul, I’m glad to see you’ve begun to expand on your tweet of the other evening. However, what I’m really interested in is: how would you do it?

ie: given that it is a legitimate question to ask “how much of the recent observed warming is due to human influence?”, how would you go about answering that question yourself?

(When I tweeted the link to Gavin’s RC post, you responded with what seemed to me to be a somewhat dismissive remark about it being “waffle and models” or something along those lines – I can’t seem to find your tweet to check, even though I can find my original one and Gavin’s reply to you, is your tweet still there?)

I’d be really interested to hear about how you would address the question of attribution.

Richard, thanks. The IPCC statements about ‘for the period 1750-2011’ are confusingly worded. The way you explain it ‘RF relative to 1750’ is much clearer. But this just confirms that Gavin is wrong. Fig 8.16 shows the RF as it is now, with a much wider spread for total anthro than for GHGs, which contradicts fig 10.5.

I haven’t forgotten your question and it will be the topic of the next post. I did delete a load of tweets last week, as I do fairly regularly, but I stand by the ‘waffle, restatement and models’ claim. Much of the RC post is just saying look at fig 10.5 and the error bars. Unfortunately, Clive did so!

So Fig 10.5 covers 195I-2011 whereas Fig 8.16 is from 1750 to 2011. That still doesn’t why the errors should be so much smaller for ANT in Fig 10.5 rather than 8.16. One might imagine that more data would reduce errors rather than increase them !

Meanwhile I have got my teeth into the new AR5 ICON – Fig 10 from the SPM. This is one plot simple enough to convince policy makers to take urgent action. Unfortunately I don’t think it is scientifically honest. Firstly the models seem to have shed all their uncertainties, and secondly CMIP5 models now appear to show linear response to CO2 forcing. Everyone knows that forcing should be logarithmic with CO2. Even I managed to derive that.